1. Field of the Invention
The present invention relates to ink jet recording devices such as printers, copiers, facsimile machines, word processors and plotters, and more particularly to an ink jet printing apparatus having means in the form of a moisture barrier for decreasing water evaporation from the ink.
2. Description of the Prior Art
The basic principle of an ink jet recording system is to eject a liquid or fused solid ink from a nozzle, slit, porous film or the like to make a recording on a recording material such as paper, cloth or film. For ejecting an ink, various methods have been proposed, such as a method of ejecting an ink using an electrostatic induction; namely, the so-called charge control system; a method of ejecting an ink using a piezoelectric element and an oscillation pressure; and a method of ejecting an ink using a pressure generated as a result of forming and growing bubbles by heat, the so-called thermal ink jet system. By using any of these methods, a high precision image on a recording material can be obtained.
Ink jet printing systems generally are of two types, i.e. continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is disturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or to a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with information from digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system""s ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
Another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. The droplet ejection system based upon thermally generated bubbles is commonly referred to as the xe2x80x9cbubble jetxe2x80x9d system.
In all of the various ink jet printing systems described above, the ink jet printing apparatus employs a reservoir for containing the ink. Since the inks generally used are water based (aqueous) inks, one potential issue is the fact that water is lost from the ink over time due to evaporation. Water loss can create basic disadvantages in the printing process in that the water that is lost from the ink over time causes changes in the ink properties and therefore effects printing performance characteristics. Current ink jet systems resolve the water loss problem by creating long vent paths or by using moisture barrier materials. However, these solutions are basically not the most efficient way to avoid the water loss related issues.
The above described disadvantages in a water based ink jet system are overcome by an ink jet apparatus that includes an ink reservoir for containing inks and a moisture barrier positioned between the ink in the ink reservoir and ambient air, the moisture barrier formed of microencapsulated water beads which preferentially releases water to the ambient air due to their relative proximity to the ventilation path. In this way, the water loss rate from the ink is reduced.
An ink jet printing process is also described which comprises the steps of incorporating an aqueous based ink composition into an ink jet printing apparatus that includes a reservoir for containing ink and a moisture barrier positioned between the ink in the ink reservoir and ambient air, the moisture barrier formed of microencapsulated water beads in proximity to the ventilation passage to provide a sustained release of the water to the ambient environment. Thereafter, droplets of the ink are caused to be ejected in an imagewise pattern onto a recording sheet.